struct ContextImpl -> class ContextImpl

Change-Id: I3bb63c41dab8b0dfaf891b188295a91fe9a4d5a8

internal/ceres/sparse_matrix.h

diff --git a/internal/ceres/sparse_matrix.h b/internal/ceres/sparse_matrix.h
index e665c8b..deab7b1 100644
--- a/internal/ceres/sparse_matrix.h
+++ b/internal/ceres/sparse_matrix.h
@@ -41,7 +41,7 @@
 #include "ceres/types.h"
 
 namespace ceres::internal {
-struct ContextImpl;
+class ContextImpl;
 
 // This class defines the interface for storing and manipulating
 // sparse matrices. The key property that differentiates different